The minute you swap roles in a Single crew aeroplane, you become a passenger, and the other pilot must have 90 day currency.
But that's the point. You do NOT swap roles. You remain the PIC, while the passenger is the sole manipulator of the controls.
I'm not saying it's wise to do so, particularly not without instructor training and the resulting ability to do a last-minute recovery. You also might have some explaining to do in case the passenger screws up and you do get into trouble.
But would it be legal? Can a passenger be the "sole manipulator of the controls", for the purpose of gaining/maintaining currency? And on the side: how do you log this?
neither need to be 90 day current.
In case of an instructor acting as PIC, and the student manipulating the controls to gain/maintain currency, I would assume that the instructor/PIC would need to be 90 day current. But indeed, the currency rules talk about carrying passenger and since the student is, legally speaking, not a passenger but a member of the flight crew, is there really no additional rule that says the instructor has to be 90-day current in this case?